As users experience the convenience of wireless connectivity, they are demanding increasing support. Typical applications over wireless networks include video streaming, video conferencing, distance learning, etc. Because wireless bandwidth availability is restricted, quality of service (QoS) management is increasingly important in 802.11 networks.
The original 802.11 media access control (MAC) protocol was designed with two modes of communication for wireless stations (STAs). The first mode, Distributed Coordination Function (DCF), is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), sometimes referred to as “listen before talk.” A wireless station (STA) waits for a quiet period on the network and then begins to transmit data and detect collisions. The second mode, Point Coordination Function (PCF), supports time-sensitive traffic flows. Using PCF, wireless access points (APs) periodically send beacon frames to communicate network identification and management parameters specific to the wireless local area network (WLAN). Between beacon frames, PCF splits: time into a contention period (CP) where the STAs implement a DCF protocol, and a contention-free period (CFP) where an AP coordinates access by the various STAs based on QoS requirements.
Because DCF and PCF do not differentiate between traffic types or sources, IEEE proposed enhancements to both coordination modes to facilitate QoS. These changes are intended to fulfill critical service requirements while maintaining backward-compatibility with current 802.11 standards.
Enhanced Distributed Channel Access (EDCA) introduces the concept of access classes (or traffic categories). Using EDCA, STAs try to send data after detecting that the wireless medium is idle for a set time period defined by the corresponding access class (AC). A higher-priority AC will have a shorter wait time than a lower-priority AC. While no guarantees of service are provided, EDCA establishes a probabilistic priority mechanism to allocate bandwidth based on ACs.
The IEEE 802.11e EDCA standard provides QoS differentiation by grouping traffic into four ACs, i.e., voice, video, best effort and background. Each transmission frame from the upper layers bears a priority value (0-7), which is passed down to the MAC layer. Based on the priority value, the transmission frames are mapped into the four ACs at the MAC layer. The voice (VO) AC has the highest priority; the video (VI) AC has the second highest priority; the best effort (BE) AC has the third highest priority; and the background (BK) AC has the lowest priority. Each AC has its own transmission queue and its own set of AC-sensitive medium access parameters. Traffic prioritization for the STAs uses the medium access parameters—the arbitration interframe space (AIFS) interval, contention window (CW, CWmin and CWmax), and transmission opportunity (TXOP)—to ensure that a higher priority AC has relatively more medium access opportunity than a lower priority AC.
Generally, in EDCA, AIFS is the time interval that a STA must sense the wireless medium to be idle before invoking a backoff mechanism or transmission. A higher priority AC uses a smaller AIFS interval. The contention window (CW, CWmin and CWmax) indicates the number of backoff time slots until the STA can attempt another transmission. The contention window is selected as a random backoff number of slots between 0 and CW. CW starts at CWmin. CW is essentially doubled every time a transmission fails until CW reaches its maximum value CWmax. Then, CW maintains this maximum value CWmax until the transmission exceeds a retry limit. A higher priority AC uses smaller CWmin and CWmax. A lower priority AC uses larger CWmin and CWmax. The TXOP indicates the maximum duration that an AC can be allowed to transmit frames after acquiring access to the medium. To save contention overhead, multiple transmission frames can be transmitted within one TXOP without additional contention, as long as the total transmission time does not exceed the TXOP duration. The active STA separates each transmission frame by a short interframe space (SIFS), which is shorter than AIFS.
To reduce the probability of two STAs colliding, because the two STAs cannot hear each other, the standard defines a virtual carrier sense mechanism. Before a STA initiates a transaction, the STA first transmits a short control frame called RTS (Request To Send), which includes the source address; the destination address and the duration of the upcoming transaction (i.e. the data frame and the respective ACK). Then, the destination STA responds (if the medium is free) with a responsive control frame called CTS (Clear to Send), which includes the same duration information. All STAs receiving either the RTS and/or the CTS set a virtual carrier sense indicator, i.e., the network allocation vector (NAV), for the given duration, and use the NAV together with the physical carrier sense when sensing the medium as idle or busy. This mechanism reduces the probability of a collision in the receiver area by a STA that is “hidden” from the transmitter STA to the short duration of the RTS transmission, because the STA hears the CTS and “reserves” the medium as busy until the end of the transaction. The duration information in the RTS also protects the transmitter area from collisions during the ACK from STAs that are out of range of the acknowledging STA. Due to the fact that the RTS and CTS are short, the mechanism reduces the overhead of collisions, since these transmission frames are recognized more quickly than if the whole data transmission frame was to be transmitted (assuming the data frame is bigger than RTS). The standard allows for short data transmission frames, i.e., those shorter than an RTS Threshold, to be transmitted without the RTS/CTS transaction.
With these medium access parameters, EDCA generally works in the following manner:
Before a transmitting STA can initiate any transmission, the transmitting STA must first sense the channel idle (physically and virtually) for at least an AIFS time interval. If the channel is idle after the initial AIFS interval, then the transmitting STA initiates an RTS transmission and awaits a CTS transmission from the receiving STA.
If a collision occurs during the RTS transmission or if CTS is not received, then the transmitting STA invokes a backoff procedure using a backoff counter to count down a random number of backoff time slots selected between 0 and CW (initially set to CWmin). The transmitting STA decrements the backoff counter by one as long as the channel is sensed to be idle. If the transmitting STA senses the channel to be busy at any time during the backoff procedure, the transmitting STA suspends its current backoff procedure and freezes its backoff counter until the channel is sensed to be idle for an AIFS interval again. Then, if the channel is still idle, the transmitting STA resumes decrementing its remaining backoff counter.
Once the backoff counter reaches zero, the transmitting STA initiates an RTS transmission and awaits a CTS transmission from the receiving STA. If a collision occurs during the RTS transmission or CTS is not received, then the transmitting STA invokes another backoff procedure, possibly increasing the size of CW. That is, as stated above, after each unsuccessful transmission, CW is essentially doubled until it reaches CWmax. After a successful transmission, CW returns to its default value of CWmin. During the transaction, the STA can initiate multiple frame transmissions without additional contention as long as the total transmission time does not exceed the TXOP duration.
The level of QoS control for each AC is determined by the combination of the medium access parameters and the number of competing STAs in the network.
The access point (AP) follows generally the same protocol described above, except that instead of employing an AIFS interval, the AP employs a PCF interframe space (PIFS) interval before attempting access to the wireless medium. FIG. 1 is a timing diagram illustrating the SIFS interval, the PIFS interval, and various AIFS intervals. As stated above, the SIFS interval is the time period between packet transmissions of a single STA or AP during a single TXOP. The PIFS interval is the time period that the AP must wait for the medium to remain idle before it initiates a transmission. The PIFS interval is typically defined as the SIFS plus a single slot time, thus assuring that the AP does not initiate a transmission during a STA's TXOP. The shortest possible AIFS interval is defined to be greater than the PIFS interval, thus providing the AP more priority to the wireless medium than the STAs.
While distributed channel access is attractive because of its more practical distributed implementation, it suffers from packet collisions which tend to increase as more STAs try to gain access to the channel. However, the EDCA MAC does not utilize channel efficiently as each STA can transmit only after it gains channel access following contention procedures. This leads to unnecessary contention among STAs belonging to the same access category (AC). Also, different ACs don't preclude the chance of a low priority packet colliding with a higher priority packet.
Example prior art references include the following:                1. U.S. Pat. No. 5,892,754 To Kompella et al.;        2. U.S. Pat. No. 6,965,933 To Haartsen;        3. U.S. Patent Application Publication No. 2004/0264397 to Benvesiste;        4. U.S. Patent Application Publication No. 2005/0036448 to Leeuwen;        5. U.S. Patent Application Publication No. 2006/0227801 to Nanda et al.;        6. IEEE 802.11 WG, “Draft Supplement to Part 11: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Medium Access Control (MAC) Enhancements for Quality of Service (QoS)”, IEEE 802.11e, November 2005; and        7. C. Lim and C-H. Choi, “TDM-based coordination function (TCF) in WLAN for high throughput”, IEEE Global Telecommunications Conference, Vol. 5, pp. 3235-3239, November 2004.        